This invention relates to the field of filtering electromagnetic energy in the microwave region in connection with a high temperature superconductor in certain configurations of microwave frequency resonator-filter combinations. Superconductive materials and particularly the recently developed high temperature superconductor (HTS) offer potential advantages when used in connection with microwave components such as filters and multiplexers. Among the primary advantage is a potential for substantial decrease in insertion loss. In specific applications, such as satellite payload applications, the potential for improvement must be weighed against the disadvantage of increasingly-complicated thermal design to provide the required cooling. What is needed is a new type of microwave filter design which can provide significant reductions in size and weight sufficient to justify the added complication of cooling.
The following references have been noted as a potentially relevant to the subject invention:
Carr, "Potential Microwave Applications of High Temperature Superconductors", Microwave Journal, December 1987, pp. 91-94. This paper discusses some of the advantages of using superconductors and microwave structures. One of the advantages is lower loss. Notwithstanding, there is nothing that suggests the structure of the present invention.
Braginski et al. "Prospects for Thin-film Electronic Devices Using High-T.sub.c Superconductors", 5th International Workshop on Future Electron Devices, Jun. 2-4, 1988, MiyagiZao, pp. 171-179. This paper discusses HTS technologies with representative device high frequency transmission strip lines, resonators and inductors. It also highlights in general terms alternative processes for the film fabrication. It doesn't address the structures themselves and how they might be employed in a specific resonator structure.
Zahopoulos et .la , "Performance of a Fully Superconductive Microwave Cavity Made of the High T.sub.c Superconductor Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y ", Applied Physics Letters, Vol. 52(25), 20 Jun. 1988, pp. 2168-2170. This paper describes a cavity fabricated with high temperature superconductive materials. The resonator employs a medium dielectric constant resonator which substantially fills a conductive cavity in a experimental structure. There is no way to tune the resonator because it is a fully enclosed structure, so it is not functional as a resonator. There are no teachings as to how to use a dielectric resonator within a cavity where the cavity itself is not fully superconductive.
U.S. Pat. Nos. 4,453,146, 4,489,293 and 4,692,723 are representative of work done on behalf of the predecessor to the assignee of the present invention. They describe various narrow band dielectric resonator/filters. There is no suggestion whatsoever in these patents of how to make effective use of superconductive materials as a wall or a portion of wall cavity.
Dworsky, U.S. Pat. No. 4,918,050 issued Apr. 17, 1990. This patent describes a reduced size superconductive resonator including high temperature superconductors. This patent describes a TEM mode resonator in which the cavity is constructed of superconductive material wherein a finger of the superconductive material extends within the wall of the cavity, and in which the cavity itself is filled with a high dielectric constant material. Since this is a TEM or quasi-TEM mode resonator, its structure cannot be readily compared to a TE mode structure.
Cohn et al., U.S. Pat. No. 4,918,049 issued Apr. 17, 1990. This patent discloses a microwave/far infrared cavity and waveguide using high temperature superconductors. Therein, a cylindrical cavity with an input and an output is provided with an inner wall composed of superconductive material. In one strip line structure, a low-loss dielectric is enclosed within a cavity with a superconductive wall and a superconductive strip mounted on a low-loss dielectric material overlying a superconducting ground plane or a conventional ground plane. The structure is substantially different than anything disclosed in the present application.
In addition to the foregoing, it is believed that a number of research groups are developing waveguide cavities in which HTS materials line the waveguide cavities or the waveguide cavities are constructed entirely of HTS. While considerable reduction in size is possible with this technology, the size of filters constructed in accordance with such a method is excessively large. Moreover, current technology does not allow the deposition as HTS thin films on any suitable cavity material. As a result, current cavities are typically made for bulk material which is typically only somewhat better than copper at best. Therefore, applications are expected to be limited to those areas where loses are very costly and small size is not desirable in the operating environment.
It has been known to make use of high-dielectric constant ceramics as resonators within waveguide cavities to allow size reduction of the resonator cavities. Placement of dielectric resonators within a waveguide cavity has in the past required that the resonator be supported at or near the center of the cavity or at least between the side walls of the cavity, which militates against substantial size reduction of the cavity. It is worthwhile to explore structures which would allow still further size reduction.